There is quite a known principle in the art of optical telecommunication networks, to detect a fiber cut by analyzing power of a signal transmitted over the span in which the fiber cut may occur. The signal may be, for example, an optical carrier wavelength assigned to a particular optical channel, or a plurality of optical carrier wavelengths transmitted over one and the same optical fiber.
According to another known approach, a pilot tone is added to the data transmitted over a fiber path in the network. Absence of the pilot tone at a particular location of the network manifests the presence of a fiber cut before this particular location.
U.S. Pat. No. 6,115,154 describes a method and a system for detecting fiber cuts in an optical network regardless of the number of EDEAs (amplifiers) that are located between the fiber cut and the monitor point. The power of a marker wavelength is compared to the power of a nearby spectral region. Where the comparison indicates that the power ratio is approximately equal to unity, a flag is raised indicating that there is a fiber cut.
U.S. Pat. No. 6,285,475 B1 proposes detecting optical faults by tapping a portion of a data signal from along a fiber network link at an optical switching site or node. A sub-carrier modulation signal can be added to the high rate data signal prior to transport over the link. The sub-carrier signal is significantly lower in both frequency and amplitude than the main data signal so as to not impact reliable reception of the main data signal. A low-pass filter tuned to the sub-carrier modulation signal filters the tapped data signal. By detecting the sub-carrier data signal, fault can be determined inexpensively and reliably in the presence of optical noise.
The problem of the above solutions is that neither of them enables precise location of a fiber cut, i.e., pointing out a specific segment of the network where a failure occurred.
The above problem, in particular, stems from the fact that any optical network comprises add-drop network elements. Any optical carrier wavelength or a pilot sub-carrier tone may disappear just because one or more optical channels marked by them are dropped by an OADM (Optical Add Drop Multiplexer); due to that detection of a fiber cut becomes problematic.